Low-latency audio output with variable group delay

ABSTRACT

A system may include an input configured to receive a digital audio input signal having at least four and fewer than 65,000 quantization levels and sampled at at least 500 kilohertz, a low-pass filter configured to receive the digital audio input signal and perform filtering on the digital audio input signal to generate a filtered digital audio input signal having a bandwidth of between approximately 100 hertz and 10 kilohertz, a digital-to-analog converter configured to receive the filtered digital audio input signal and convert the filtered digital audio input signal into an equivalent analog audio input signal, and a driver configured to receive the equivalent analog audio input signal and drive an analog audio output signal to a transducer, wherein a group delay from the input to an output of the driver is less than 50 microseconds.

RELATED APPLICATIONS

The present disclosure claims priority to U.S. Provisional PatentApplication Ser. No. 62/858,667, filed Jun. 7, 2019, and U.S.Provisional Patent Application Ser. No. 62/870,152 filed Jul. 3, 2019,both of which are incorporated by reference herein in their entirety.

FIELD OF DISCLOSURE

The present disclosure relates in general to circuits for electronicdevices, including without limitation personal audio devices such aswireless telephones and media players, and more specifically, toproviding a low-latency audio output path with a variable group delay inorder to optimize filtering of out-of-band noise from the audio outputpath.

BACKGROUND

Personal audio devices, including wireless telephones, such asmobile/cellular telephones, cordless telephones, mp3 players, and otherconsumer audio devices, are in widespread use. Such personal audiodevices may include circuitry for driving a pair of headphones or one ormore speakers. Such circuitry often includes a speaker driver includinga power amplifier for driving an audio output signal to headphones orspeakers.

In many audio output systems, it is desirable to have low latency,wherein latency may be defined as a duration of time it takes a digitalaudio input signal to be processed and reproduced as audible sound by atransducer. An example of a scenario in which low latency is desired isnoise cancellation using one or more microphones in a feedback or feedforward system. It is often desirable for such audio system to operateat a high dynamic range with low power. Many will be battery operatedand operate under a wide range of audio levels.

Out-of-band noise (e.g., noise at ultrasonic frequencies) present withinan audio signal may cause undesirable dissipation of power in drivers,amplifiers, and other components of an audio system. Out-of-band noisemay also spectrally fold into the audible audio band, leading toundesirable noise. Accordingly, to maximize dynamic range and minimizepower, it may be desirable to reduce or eliminate out-of-band noise froman audio signal using out-of-band filtering. However, out-of-bandfiltering may introduce undesirable delay into an audio path.

SUMMARY

In accordance with the teachings of the present disclosure, one or moredisadvantages and problems associated with existing approaches toout-of-band noise filtering may be reduced or eliminated.

In accordance with embodiments of the present disclosure, a system mayinclude a filter configured to receive a digital audio input signalquantized at between two and 257 quantization levels and sampled at atleast 500 kilohertz, the filter further configured to perform filteringon the digital audio input signal to generate a filtered digital audioinput signal, the filter having a selectable variable group delay, adigital-to-analog converter configured to receive the filtered digitalaudio input signal and convert the filtered digital audio input signalinto an equivalent analog audio input signal, and a driver configured toreceive the equivalent analog audio input signal and drive an analogaudio output signal to a transducer.

In accordance with these and other embodiments of the presentdisclosure, a method may include receiving a digital audio input signalquantized at between two and 257 quantization levels and sampled at atleast 500 kilohertz, filtering, with a filter having a selectablevariable group delay, the digital audio input signal to generate afiltered digital audio input signal, converting the filtered digitalaudio input signal into an equivalent analog audio input signal with adigital-to-analog converter, and driving an analog audio output signalbased on the equivalent analog audio input signal to a transducer.

In accordance with these and other embodiments of the presentdisclosure, a system may include an input configured to receive adigital audio input signal having at least four and fewer than 65,000quantization levels and sampled at at least 500 kilohertz, a low-passfilter configured to receive the digital audio input signal and performfiltering on the digital audio input signal to generate a filtereddigital audio input signal having a bandwidth of between approximately100 hertz and 10 kilohertz, a digital-to-analog converter configured toreceive the filtered digital audio input signal and convert the filtereddigital audio input signal into an equivalent analog audio input signal,and a driver configured to receive the equivalent analog audio inputsignal and drive an analog audio output signal to a transducer, whereina group delay from the input to an output of the driver is less than 50microseconds.

In accordance with these and other embodiments of the presentdisclosure, a method may include receiving a digital audio input signalhaving at least four and fewer than 65,000 quantization levels andsampled at at least 500 kilohertz, filtering the digital audio inputsignal with a low-pass to generate a filtered digital audio input signalhaving a bandwidth of between approximately 100 hertz and 10 kilohertz,converting the filtered digital audio input signal into an equivalentanalog audio input signal with a digital-to-analog converter, anddriving an analog audio output signal to a transducer based on theequivalent analog audio input signal, wherein a group delay from theinput to an output of the driver is less than 50 microseconds.

Technical advantages of the present disclosure may be readily apparentto one skilled in the art from the figures, description and claimsincluded herein. The objects and advantages of the embodiments will berealized and achieved at least by the elements, features, andcombinations particularly pointed out in the claims.

It is to be understood that both the foregoing general description andthe following detailed description are examples and explanatory and arenot restrictive of the claims set forth in this disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete understanding of the present embodiments and advantagesthereof may be acquired by referring to the following description takenin conjunction with the accompanying drawings, in which like referencenumbers indicate like features, and wherein:

FIG. 1 illustrates an example personal audio device, in accordance withembodiments of the present disclosure;

FIG. 2 illustrates a block diagram of selected components of an exampleaudio integrated circuit of a personal audio device, in accordance withembodiments of the present disclosure; and

FIG. 3 illustrates a block diagram of selected components of an examplevariable latency out-of-band noise filter, in accordance withembodiments of the present disclosure.

DETAILED DESCRIPTION

FIG. 1 illustrates an example personal audio device 1, in accordancewith embodiments of the present disclosure. FIG. 1 depicts personalaudio device 1 coupled to a headset 3 in the form of a pair of earbudspeakers 8A and 8B. Headset 3 depicted in FIG. 1 is merely an example,and it is understood that personal audio device 1 may be used inconnection with a variety of audio transducers, including withoutlimitation, headphones, earbuds, in-ear earphones, and externalspeakers. A plug 4 may provide for connection of headset 3 to anelectrical terminal of personal audio device 1. Personal audio device 1may provide a display to a user and receive user input using a touchscreen 2, or alternatively, a standard liquid crystal display (LCD) maybe combined with various buttons, sliders, and/or dials disposed on theface and/or sides of personal audio device 1. As also shown in FIG. 1,personal audio device 1 may include an audio integrated circuit (IC) 9for generating an analog audio signal for transmission to headset 3and/or another audio transducer.

FIG. 2 illustrates a block diagram of selected components of an exampleaudio IC 9 of a personal audio device, in accordance with embodiments ofthe present disclosure. As shown in FIG. 2, a microcontroller core 18may supply a digital audio input signal DIG_IN, which may comprise amodulated digital signal, such as a pulse-density modulated signal, forexample. To have low latency between a digital audio input signal DIG_INand output signal V_(OUT), a high, oversampled rate (e.g., 500 KHz to 6MHz) may be desired. In some embodiments, also to maintain lowerout-of-band noise, digital audio input signal DIG_IN may comprise aquantized digital signal with between four and 65,000 quantizationlevels. In particular embodiments, digital audio input signal DIG_IN maycomprise a quantized digital signal with at least sixteen quantizationlevels. In other particular embodiments, digital audio input signal DIG_IN may comprise a quantized digital signal having between two and 257quantization levels.

A variable delay out-of-band filter 10 may receive digital audio inputsignal DIG_IN and filter digital audio input signal DIG_IN in accordancewith filter parameters of variable delay out-of-band filter 10 in orderto generate filtered digital audio input signal DIG_IN′. In someembodiments, variable delay out-of-band filter 10 may be configured togenerate filtered digital audio input signal DIG_IN′ having a signalbandwidth between approximately 10 hertz and 10 kilohertz. Variabledelay out-of-band filter 10 may comprise a low-pass filter having one ormore filter characteristics (e.g., corner frequency, group delay,latency, filter order) which may be varied in response to one or morecontrol signals CONTROL received from control circuit 20. Variable delayout-of-band filter 10 may comprise a finite impulse response filter oran infinite impulse response filter.

Control circuit 20 may receive digital audio input signal DIG_IN andbased on one or more characteristics of digital audio input signalDIG_IN (e.g., signal magnitude of digital audio input signal DIG_IN),generate one or more control signals CONTROL for controlling one or morefilter characteristics of variable delay out-of-band filter 10, asdescribed in greater detail below. In addition, control circuit 20 maygenerate one or more control signals CONTROL for controlling one or morefilter characteristics of variable delay out-of-band filter 10 based onwhether digital audio input signal DIG_IN is being used for real-timefeedforward or real-time feedback processing (e.g., as would be the casewhen digital audio input signal DIG_IN is used in an active noisecancellation application).

Although FIG. 2 depicts control circuit 20 generating one or morecontrol signals based on one or more characteristics of digital audioinput signal DIG_IN, in other embodiments, control circuit 20 maygenerate one or more control signals CONTROL based on other factors(e.g., user-selected settings).

As shown in FIG. 2, audio IC 9 may also include a digital-to-analogconverter (DAC) 14, which may convert filtered digital audio inputsignal DIG_IN′ to an analog signal V_(IN). DAC 14 may supply analogsignal V_(IN) to an amplifier 16 which may amplify or attenuate audioinput signal V_(IN) to provide an audio output signal V_(OUT), which mayoperate a speaker, a headphone transducer, a line level signal output,and/or other suitable output. In some embodiments, amplifier 16 maydrive audio output signal V_(OUT) at a voltage of between approximatelyone milliwatt and approximately 100 milliwatts to an output transducerhaving an impedance of between approximately two ohms and approximately100 ohms. Although shown in FIG. 2 as a differential signal, in someembodiments, audio output signal V_(OUT) may be a single-ended signal.In some embodiments, DAC 14 may be an integral component of amplifier16.

Although FIGS. 1 and 2 contemplate that audio IC 9 resides in a personalaudio device, systems and methods described herein may also be appliedto electrical and electronic systems and devices other than a personalaudio device, including audio systems for use in a computing devicelarger than a personal audio device, an automobile, a building, or otherstructure.

FIG. 3 illustrates a block diagram of selected components of an examplevariable latency out-of-band noise filter 10, in accordance withembodiments of the present disclosure. In some embodiments, variablelatency out-of-band noise filter 10 may be used to implement variablelatency out-of-band noise filter 10 shown in FIG. 2.

As shown in FIG. 3, variable latency out-of-band noise filter 10 maycomprise a plurality of individual filter elements 30 (e.g., filterelements 30A, 30B, . . . , 30N), coupled in series fashion such that anoutput of one filter element 30 may serve as an input of a subsequentfilter element 30, wherein first filter element 30A may receive digitalaudio input signal DIG_IN, perform low-pass filtering on digital audioinput signal DIG_IN, and output such filtered signal to the input ofsecond filter element 30B and an input of multiplexer 32. Similarly,second filter element 30B may receive digital audio input signal DIG_IN,perform further low-pass filtering on digital audio input signal DIG_INas filtered by first filter element 30A, and output such filtered signalto the input of a third filter element 30 and an input of multiplexer32. All subsequent filter elements 30 may receive the output of thefilter element 30 that comes before it, communicating its result to theinput of multiplexer 32 and (except for the final filter 30N) to anothersubsequent filter element 30.

A state machine 34 may receive one or more control signals from controlcircuit 20, and further process such one or more control signals inorder to control multiplexer 32 to select a desired filter elementoutput to be output as filtered digital audio input signal DIG_IN′. Ifrobust filtering is desired, an output from a later stage filter element30 (e.g., higher-order filtering) may be selected, at the cost ofincreased latency. If robust filtering is not desired, an output from anearlier stage filter element 30 (e.g., lower-order filtering) may beselected. For example, for higher magnitude signals which may bettermask out-of-band noise, the one or more control signals from controlcircuit 20 may cause variable latency out-of-band noise filter 10 tooperate with lower-order filtering (e.g, causing multiplexer 32 toselect the output of filter element 30A). Conversely, for lowermagnitude signals which may not mask out-of-band noise, the one or morecontrol signals from control circuit 20 may cause variable latencyout-of-band noise filter 10 to operate with higher-order filtering (e.g,causing multiplexer 32 to select the output of filter element 30N).

State machine 34 may also be configured to control switching among delaymodes of variable latency out-of-band noise filter 10 at particularpoints of time in order to reduce switching artifacts (e.g., audiblepops and clicks) that might otherwise occur. For example, by receivingdigital audio input signal DIG_IN as an input signal, state machine 34may be configured to switch among delay modes at or proximate to zerocrossings of digital audio input signal DIG_IN. As selection is changedamong the outputs of filter elements 30, state machine 34 may alsoimplement a cross-fading approach that slowly transitions betweenselections (e.g., by slowly decreasing a weight of a de-selected modewhile slowly increasing a weight of a selected mode). In addition oralternatively, state machine 34 may slowly change coefficients whenswitching from one delay mode to another delay mode.

Although FIG. 3 shows control signals being used to select an order (andthus a latency) of variable latency out-of-band noise filter 10, in someembodiments, the one or more control signals CONTROL received byvariable latency out-of-band noise filter 10 may control a cut offfrequency applied by the filtering of filter elements 30 (e.g., a cutofffrequency of 250 KHz for high signal magnitudes, reducing to 25 KHz forlower signal magnitudes). Further, in some embodiments, the one or morecontrol signals CONTROL received by variable latency out-of-band noisefilter 10 may control other characteristics of variable latencyout-of-band noise filter 10 and/or filter elements 30 besides order,latency, and corner frequency.

Although FIG. 3 represents an architecture for some embodiments ofvariable latency out-of-band noise filter 10, the present disclosureexpressly contemplates other architectures for providing a variablelatency out-of-band noise filter having one or more characteristicsbased on one or more characteristics of an audio signal. For example,variable latency out-of-band noise filter 10 may be implemented withmultiple filters with a cross-fade of outputs of the multiple filters asshown in FIG. 3. However, other architectures may, in addition to or inlieu of the architecture described above, implement a filter withtime-varying coefficients, implement a filter with varying structure,and/or implement any other suitable features.

Using the methods and systems herein may enable a small group delay(e.g., less than 50 μs) from the input of variable latency out-of-bandnoise filter 10 and the output of amplifier 16.

As used herein, when two or more elements are referred to as “coupled”to one another, such term indicates that such two or more elements arein electronic communication or mechanical communication, as applicable,whether connected indirectly or directly, with or without interveningelements.

This disclosure encompasses all changes, substitutions, variations,alterations, and modifications to the example embodiments herein that aperson having ordinary skill in the art would comprehend. Similarly,where appropriate, the appended claims encompass all changes,substitutions, variations, alterations, and modifications to the exampleembodiments herein that a person having ordinary skill in the art wouldcomprehend. Moreover, reference in the appended claims to an apparatusor system or a component of an apparatus or system being adapted to,arranged to, capable of, configured to, enabled to, operable to, oroperative to perform a particular function encompasses that apparatus,system, or component, whether or not it or that particular function isactivated, turned on, or unlocked, as long as that apparatus, system, orcomponent is so adapted, arranged, capable, configured, enabled,operable, or operative. Accordingly, modifications, additions, oromissions may be made to the systems, apparatuses, and methods describedherein without departing from the scope of the disclosure. For example,the components of the systems and apparatuses may be integrated orseparated. Moreover, the operations of the systems and apparatusesdisclosed herein may be performed by more, fewer, or other componentsand the methods described may include more, fewer, or other steps.Additionally, steps may be performed in any suitable order. As used inthis document, “each” refers to each member of a set or each member of asubset of a set.

Although exemplary embodiments are illustrated in the figures anddescribed below, the principles of the present disclosure may beimplemented using any number of techniques, whether currently known ornot. The present disclosure should in no way be limited to the exemplaryimplementations and techniques illustrated in the drawings and describedabove.

Unless otherwise specifically noted, articles depicted in the drawingsare not necessarily drawn to scale.

All examples and conditional language recited herein are intended forpedagogical objects to aid the reader in understanding the disclosureand the concepts contributed by the inventor to furthering the art, andare construed as being without limitation to such specifically recitedexamples and conditions. Although embodiments of the present disclosurehave been described in detail, it should be understood that variouschanges, substitutions, and alterations could be made hereto withoutdeparting from the spirit and scope of the disclosure.

Although specific advantages have been enumerated above, variousembodiments may include some, none, or all of the enumerated advantages.Additionally, other technical advantages may become readily apparent toone of ordinary skill in the art after review of the foregoing figuresand description.

To aid the Patent Office and any readers of any patent issued on thisapplication in interpreting the claims appended hereto, applicants wishto note that they do not intend any of the appended claims or claimelements to invoke 35 U.S.C. § 112 (f) unless the words “means for” or“step for” are explicitly used in the particular claim.

What is claimed is:
 1. A system, comprising: an input configured toreceive a digital audio input signal having at least four and fewer than65,000 quantization levels and sampled at at least 500 kilohertz; alow-pass filter configured to receive the digital audio input signal andperform filtering on the digital audio input signal to generate afiltered digital audio input signal having a bandwidth of betweenapproximately 100 hertz and 10 kilohertz; a digital-to-analog converterconfigured to receive the filtered digital audio input signal andconvert the filtered digital audio input signal into an equivalentanalog audio input signal; and a driver configured to receive theequivalent analog audio input signal and drive an analog audio outputsignal to a transducer; wherein a group delay from the input to anoutput of the driver is less than 50 microseconds.
 2. The system ofclaim 1, wherein the low-pass filter has a variable group delay.
 3. Thesystem of claim 2, wherein the variable group delay is selected based onone or more signal characteristics of the digital audio input signal. 4.The system of claim 3, wherein the one or more signal characteristics ofthe digital audio input signal comprise a magnitude of the digital audioinput signal.
 5. The system of claim 2, wherein the variable group delayis selected based on whether the digital audio input signal is beingused for real-time feedback or real-time feedforward processing.
 6. Thesystem of claim 1, wherein the filter is configured to switch between afirst mode in which the filter has a first variable group delay and asecond mode in which the filter has a second variable group delay in amanner that minimizes audible audio artifacts associated with suchswitching.
 7. A method, comprising: receiving a digital audio inputsignal having at least four and fewer than 65,000 quantization levelsand sampled at at least 500 kilohertz; filtering the digital audio inputsignal with a low-pass to generate a filtered digital audio input signalhaving a bandwidth of between approximately 100 hertz and 10 kilohertz;converting the filtered digital audio input signal into an equivalentanalog audio input signal with a digital-to-analog converter; anddriving an analog audio output signal to a transducer based on theequivalent analog audio input signal; wherein a group delay from theinput to an output of the driver is less than 50 microseconds.
 8. Themethod of claim 7, wherein the low-pass filter has a variable groupdelay.
 9. The method of claim 8, further comprising selecting thevariable group delay based on one or more signal characteristics of thedigital audio input signal.
 10. The method of claim 9, wherein the oneor more signal characteristics of the digital audio input signalcomprise a magnitude of the digital audio input signal.
 11. The methodof claim 8, further comprising selecting the variable group delay basedon whether the digital audio input signal is being used for real-timefeedback or real-time feedforward processing.
 12. The method of claim 7,further comprising switching between a first mode in which the filterhas a first variable group delay and a second mode in which the filterhas a second variable group delay in a manner that minimizes audibleaudio artifacts associated with such switching.